Cardiac performance and resistance to flow in vascular beds are subject to tonic and phasic activity of autonomic effectors. Afferent input from arterial baroreceptors are a major determinent of autonomic effector activity. Changes produced by various inputs other than the arterial baroreceptors, can be considered factors capable of influencing both the set point and the gain of the baroreceptor reflex. Resetting of the blood pressure control system is of key importance in integrated cardiovascular control and the subject of this study. In an open-loop system various carotid sinus pressures will be used to set the vascular resistance in an isolated perfused gastrocnemius-soleus muscle, hind paw and kidney. The integrated effects of activity from suprapontine sites and varying degrees of baroreceptor input in modifying the active and passive dilator mechanisms are measured. The method permits analysis of altered vascular parameters not only at high and low sinus pressure but at intermediate points throughout the reflex. To further characterize this interaction, microelectrodes will be used to identify units in the medullary areas of the nucleus solitarius which respond to sinus nerve afferent activity. Analysis of inhibitory and facilitory interaction of these neurons with extra-medullary systems, e.g., the vasodilator area of the hypothalamus, will yield more detailed information on the way in which the medullary vasomotor area mediates cardiovascular adjustments. It is the interaction of information arriving from baroreceptors with that of other inputs that provides the blood pressure control system which has flexability to guide the heart and blood vessels through ever changing conditions that man meets daily. BIBLIOGRAPHIC REFERENCES: Fuller, E. O., P. M. Gallette, J. W. Manning and D. O. Nutter, A perfused uterine preparation for the study of uterine and fetal physiology. Circulation in the Fetus and Newborn. New York: Raven Press. 1976. Putnam, S. J. and J. W. Manning., Repetitively firing medullary neurons responsive to hypothalamic and carotid sinus nerve activation. Fed. Proceed. 35:240, 1976.